1. Field of the Invention
The present invention relates to a digital camera including an image pick-up device such as a CCD image sensor, and more specifically relates to such a camera having a structure preventing defocusing which may be caused by heat produced by the image pick-up device and/or ambient temperature variations of the camera from occurring.
2. Description of the Related Art
In digital cameras including an image pick-up device such as a CCD image sensor, a mirror-box fixing structure wherein a main frame in a camera body is made of a material having a high stiffness such as a stainless steel, a resin-molded or die-cast mirror box is fixed to the front of the main frame, and the image pick-up device is fixed to the back of the mirror box, has been proposed for miniaturization and weight reduction of the camera. FIG. 8A shows a schematic view of a digital camera having such a mirror-box fixing structure. This digital camera is provided with a main frame 110 having an L-shaped vertical cross section. The main frame 110 is composed of a back plate 111 and a bottom plate 112 which are made of stainless steel plate and formed separately or integrally. A die-cast mirror box assembly 120 is fixed on the main frame 110 to be supported thereby. The mirror box assembly 120 is constructed so that various kinds of components such as a mirror box 121 and a pentagonal prism 123 are mounted to the mirror box assembly 120 and so that a lens mount 102 to which a photographing lens (not shown) is detachably attached is provided on the front of the mirror box 121. In addition, the main frame 110 is provided on the back plate 111 with an aperture 111a through which the interior of the mirror box assembly 120 is exposed to the outside on the rear side thereof, and an image pick-up device 130 is fixed to a rear surface of the back plate 111 to face the aperture 111a. This arrangement makes it possible to miniaturize the camera and reduce weight of the camera while increasing the strength of the camera body by a greater degree than in the case where the whole camera body is die-cast of aluminum or molded out of resin.
In digital cameras using a CCD image sensor as an image pick-up device, the temperature of the CCD image sensor rises excessively due to the driving current supplied thereto if the CCD image sensor operates continuously. This temperature rise increases the dark current in the CCD image sensor, thus causing an increase in noise of the image captured by the CCD image sensor. To prevent this problem of image deterioration from occurring, a technology for making the heat that is produced by the CCD image sensor transferred to the camera body to dissipate the heat therefrom has been proposed in Japanese laid-open patent publication No. 2003-69886.
The digital camera shown in FIG. 8A has adopted such a conventional technology; the heat produced by the CCD image sensor 130 is transferred to the main frame 110 to be dissipated from the main frame 110 so that the temperature of the CCD image sensor is prevented from rising excessively to thereby prevent image quality from deteriorating. However, if the temperature of the main frame 110 rises by the heat which is produced by the CCD image sensor 130 and transferred from the CCD image sensor 130 to the main frame 110 and/or by ambient temperature variations, heat expansion of the main frame 110 occurs both in a direction of the thickness of the main frame and in a direction along a surface thereof as shown by chain lines in FIG. 8B. At the same time, the heat transferred to the main frame 110 is partly transferred to the mirror box assembly 120 that is fixed to the main frame 110. Die-cast aluminum or molded resin which is used as a material of the mirror box 121 has a greater thermal conductivity than the main frame 110, and therefore not only the main frame 110 but also the mirror box 121 expand by the heat transferred from the CCD image sensor 130 via the main frame 110. Therefore, heat expansion of each of the main frame 110 and the mirror box assembly 120 occurs in the optical axis direction (horizontal direction as viewed in FIGS. 8A and 8B). Specifically, the imaging surface (front surface) of the CCD image sensor 130 deviates rearward in the optical axis direction from its original position by an amount of movement corresponding to the thermal expansion of the thickness of the main frame 110 in the optical axis direction in the main frame 110, while the front surface (lens mounting surface) of the lens mount 102 moves forward in the optical axis direction by an amount of movement corresponding to the thermal expansion of the mirror box 121 in the optical axis direction in the mirror box assembly 120. As a result of these positional deviations, the distance between the imaging surface of the CCD image sensor 130 and the front surface of the lens mount 102 in the optical axis direction varies depending on the heat transferred to the main frame 110 and the mirror box 121, and accordingly, defocusing occurs in the image focused on the CCD image sensor 130.
In addition, a bottom portion of the mirror box assembly 120 is fixed to the bottom wall 112 by set screws and the like, and is therefore restrained from expanding thermally by a greater force than an upper portion of the mirror box assembly 120 so that the front surface of the lens mount 102 tilts relative to a plane orthogonal to a photographing optical axis O depending on a degree of the thermal expansion of the mirror box assembly 120 as shown in FIG. 8B. Consequently, the photographing optical axis O tilts downwards, and accordingly defocusing partially occurs in the image focused on the CCD image sensor 130.